Well, it is less a desire and more a thing to help you define the character. Though it is something you can make mechanics on or around. More importantly, knowing your character's desire will help you in role playing the character.
An Emotional Goal Not A Practical Goal
A character's desire is an emotional goal, not a practical goal. It is effectively an emotional reward they are seeking - that they desire if you will - and that is driving their action. It is important to note that it is an emotional goal, meaning it is not an action or physical thing that can be obtained, but rather an emotional state. It is also worth noting, and Hillfolk does this, that the simpler your desire the better.
Forgiveness, love, to be punished, validation, respect, all make wonderful simple desires.
Why Is It Our Desire
Once you have what it is the character desires, think about why it is they desire that. This helps you to define the desire, it's source, and gives a reasoning to it that works. Keep in mind that it doesn't have to be a huge or grandiose reason, it just has to be significant for the character.
For example, many a character that desires power - to be strong - does so because at one point in their life they felt weak and that is something they never wish to feel again. So even when they are ridiculously strong by most other people's standards, they still seek power because as long as they can become stronger they are not strong enough to never be made to feel weak again.
How We Pursue Our Desire Determines Good/Evil, Not The Desire
Another thing to remember is that it is how we pursue our desires that determines where we fall on the scales of good and evil as opposed to the desire itself. Both Goku and Frieza desire to be the strongest in the galaxy in Dragonball Z, but how they go about this ends up with Goku as a hero and Frieza as a villain.
How we go about pursuing our desires is the key difference between so many characters. It is the difference between issuing a challenge to the man who killed your father/teacher/whatever to a duel to the death for revenge, climbing in their window at night to assassinate them, and completely ruining them socially and financially until you own them as a slave that you torture nightly while they scream and beg for mercy.
Why Can't We Have Our Desire?
Finally, for bonus points, why can't we have our desire? This can be more internal or external, and which one you choose will determine the type of growth and experience the character will have. In the best cases I feel like you will have both, an internal and external reason that stops it, but that is up to you.
Some Examples From Popular Culture
To help you out here are some examples for how these can work.
Spider-man desires to be forgiven for the death of his uncle ben. The moral of his story that is oft repeated 'with great power comes great responsibility' is the manifestation of this desire. This is the core desire that keeps making Peter Parker put on the mask and go out to fight crime time after time despite what else he wants. Because anytime he is not doing everything he can with his power to save people, he is - once again - letting Uncle Ben be killed while he stands aside for his own petty desires. Years and years have gone by int he comic. Peter has by and large come to terms with his part in Uncle Ben's death. He has saved thousands of people in return. Even still though, if he were to stop being Spider-Man, if he were to stop saving people with everything he has, he no longer becomes worthy of that peace and absolution and so he continues on.
Bruce Wayne's core desire changes depending on who is writing him, but in the Batman stories I am more familiar with from the comics what he desires most is Control. He desires it over himself, and Gotham. He desires it so that no little boy ever has to go through what he did with the loss of his parents. He can't have this control as complete as he wants because of his need for control. His own rules, his own principles that he uses to keep himself under control prevent it. However, you can see how in alternate universes this desire manifests in a way for a more totalitarian Gotham.
Walter White in Breaking Bad wants Respect. He can't have it because of his pride. He feels that people helping him out - giving him charity - is them pitying him. He feels that his wife and kid and friends pity him because of the cancer eating his body. He wants to be respected as a man and provider, and to feel he is worthy of that respect. To do this he needs to provide for himself for his treatment, and that brings him more and more into conflicts where he always reacts adversely to not being respected as an equal - not being given the respect he feels he is due.
What is your character's desire? What is the emotional reward they want from the world? From the people around them?
And remember, the simpler the desire the better.
- To Punish
- To Be Punished
But there are lots more. And remember, with many of these things you can apply modifiers in the same vein for similar things. For example "Power" could be "Wealth" or it could be the common anime desire to be 'strong'.
Have fun with it. Keep it simple.
On Wednesday we'll talk about using our desire for practical goals.